MP33-07 CLINICAL AND METABOLIC RISK FACTORS FOR PLUG FORMATION AMONG IDIOPATHIC CALCIUM OXALATE STONE FORMERS

INTRODUCTION AND OBJECTIVES: Collecting duct plugs could provide a nidus for stone formation within the kidney. However, their role in idiopathic calcium oxalate (iCaOx) calculogenesis remains poorly defined. Thus, we evaluated factors associated with endoscopically detected plugs in a cohort of iCaOx stone formers. METHODS: Data was collected from a prospective cohort of patients undergoing percutaneous nephrolithotomy (PCNL) between 2009-2014. After stone removal, accessible renal papillae were endoscopically videotaped, biopsied, and analyzed to quantify the percent surface area (SA) occupied by plaque and plug. Stone composition was determined by micro-CT, and all patients underwent metabolic evaluation with 24 hour urine collections. RESULTS: Data were available for 89 iCaOx formers. Plugs were detected in 33 (37%), with a mean % plug SA of 1.4 (median1⁄41.0, IQR1⁄40.2-1.6). Patients with plugs had a higher mean % plaque SA than those with no plugs (5.3% versus 2.3%, p1⁄40.02); moreover, a moderately strong correlation was found between % plaque and % plug (Spearman r1⁄40.25, p-value1⁄4.016). Patients with a higher % mean SA plugging (>1%) were more likely to have a higher urinary pH (mean 6.3 vs 6.0, p1⁄40.03). Neither urinary hydroxyapatite supersaturation (SS) or percentage apatite significantly differed between the high and low plug group (mean 4.7 vs 3.5 D.G.; p1⁄40.08 and 20.7 vs 10.9; p1⁄40.40, respectively). CONCLUSIONS: Within a cohort of iCaOx stone formers, plaque and plugs often coexist and are positively correlated. Those with greater amounts of plugs also tended to have higher levels of urinary pH. Gender, hydroxyapatite SS and hydroxyapatite stone content were similar across high and low plug group. Intraluminal features known to favor calcium phosphate crystallization, namely higher pH, may play a role in plug formation. However, the pathogenic significance of these plugs in the iCaOx population remains to be established.